Cloyd has just received a letter from his sister. The problem, however, is that Cloyd is functionally illiterate and so can't read more than a few words of it.
Walter senses that there's something about the letter that's bothering Cloyd and naturally wants to find out what the problem is. So, he asks Cloyd to tell him about the letter and what news it's brought from home.
But Cloyd, feeling confused and angry at the old man, yells at Walter and tells him that it's none of his business. Walter backs off, apologizing, saying that he didn't mean to pry. But instead of placating Cloyd, his words have the exact opposite effect:
“I don't know what's in the letter!” Cloyd shouted, standing up. “I can't read it. I don't know how. There, are you satisfied?”
Cloyd is clearly upset by his not being able to read the letter. He wants to know exactly what his sister has written to him, and the fact that he can't read has made him incredibly frustrated. Not only that, he hates being made to look weak in front of Walter. Cloyd is also worried that Walter will now think that he's stupid.
As it turns out, Walter is very sympathetic and kindly offers to read the letter to him. He also offers to read mining papers to Cloyd.